"Trevor Wilson" wrote in
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"Dave Plowman (News)" wrote in
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Got an 'interesting' fault on an ancient Linsey Hood
amp. Switch on a light etc, and sometimes *one* channel
bursts into loud mains hum - but carries on working. And
the output transistors get very hot. It's not normally
susceptible to mains born interference - so any guesses
before I rip it apart?

Both channels seem to match ok before this happens.

**First off: The "Mains hum" is not mains hum.

Well, it is still mains-related.

It is 100Hz hum.

Yes, power supply ripple from a full wave rectifier.

It is the sound made by an amplifier when so
much current is consumed from the power supply, that
ripple appears on the outputs.

Agreed.

Second off: You need to ascertain if the output devices
get hot with no load connected. If they still get hot,
then look for a fault in the bias section, or further
down the chain. Something is causing the output devices
to conduct too much current.

Agreed. An open bias network component, possibly a pot is a common cause.

If the output devices run cool when no load is connected,
then it is likely you have a large DC Voltage at the
output. If so, the fault could lie anywhere in the amp.

IME large DC voltage on the output is often very detrimental to the load,
unless the load is a stout bench resistor bank.